Organic films are advantageous in that they are more flexible, less prone to rupture, and lighter in weight as compared with glass. Recently, there are increasing studies to render displays more flexible by forming substrates of flat display panels from organic films.
Generally, examples of the resin to be used for an organic film include polyester, polyamide, polyimide, polycarbonate, polyethersulfone, acrylic resin, and epoxy resin. Especially, since polyimide has higher thermal resistance than that of other resins and has both superior mechanical characteristics, such as high mechanical strength, abrasion resistance, dimensional stability and chemical resistance, and superior electric characteristics, such as insulation property, flexible substrates using polyimide films are under development.
Examples of flexible substrates include substrates for displays, such as a flexible TFT (Thin Film Transistor) substrate, a flexible organic EL element substrate, and a flexible color filter substrate. Color filters are members required in order to make an organic electro-luminescence display device or a liquid crystal display device each using white light as a light source perform color display. Three-color color filters in which three-color colored pixels, namely, red colored pixels, green colored pixels and blue colored pixels, are patterned minutely are common. In a three-color color filter, white color is produced by additive color mixing of colored pixels of three colors: red, green, and blue.
The flat panel display substrate and the color filter substrate enumerated above are required to be high in light transmission in the visible region. In order to prevent aggravation of alignment accuracy caused by heating at the time of forming a display element and a light receiving device such as a TFT and a color filter, the substrates are required to be low in coefficient of thermal expansion (CTE) and not to suffer from warpage. Moreover, in order to prevent color shift when viewed at an angle or to suppress ambient light reflection when using a circular polarizing film, the substrates are required to be low in birefringence.
As a transparent polyimide that exhibits a high light transmission in the visible region, there have been disclosed polyimides made from a fluorine-containing and/or alicyclic acid dianhydride and a fluorine-containing and/or alicyclic diamine (see, for example, PLTs 1 to 4).
Moreover, as a method for producing an organic EL device that exhibits small warpage even if there is a large difference in CTE, there has been disclosed a method of suppressing warpage of a substrate by laminating a shape-retaining layer on the side opposite from a higher CTE layer laminated on a lower CTE layer to offset the stresses generated on both sides of the lower CTE layer (see, for example, PLT 5). Furthermore, as a flexible substrate that attains both a low CTE and a low birefringence, there has been disclosed a polyimide produced by a specific method using a specific acid dianhydride and a specific diamine. (See, for example, PLT 6.)